High temperature and density plasmas are used in a variety of applications, such as in plasma torches for flame-spraying refractory materials, in sources of continium radiation and as a medium for carrying out chemical reactions. Techniques presently employed to produce continuous plasmas include the use of very high current electric arcs, high power radio-frequency (rf) or microwave discharges and very highly focused, high power continuous wave lasers. Short pulse length plasmas are commonly produced by high power pulsed lasers. A disadvantage of the electric arc and the high frequency discharge is that of having a solid material in close proximity to the plasma. This material is subjected to intense radiant and convective heating, and may erode or otherwise degrade at a rapid rate, leading to chemical contamination of the plasma. A disadvantage of the rf generated plasma is the presence of large, stray fields of rf energy. All of these techniques result in limited plasma temperature.
After ignition, a plasma tends to propagate away from the higher intensity regions of the focused laser beam, and stabilize at a location where the radiation intensity is the minimum required for plasma maintenance. A consequence of the plasma migration to a relatively low intensity region is a very much reduced plasma temperature. Additionally, the absorption of the laser radiation by the plasma is also reduced, leading to an inefficient utilization of laser energy.
Stable, mobile atmospheric air plasmas may be produced with an intense, continuous wave, CO.sub.2 laser beam. These plasmas can be initiated by interaction of the laser beam with a metallic target. The function of the target, insofar as is presently known, is to produce a sufficient free electron concentration to provide a localized region, external to the target, with a significant absorption coefficient for the laser radiation. After initiation, sufficient energy is transferred to the surrounding air, to produce ionization. The air plasma then travels up the laser beam with a velocity proportional to I/I.sub.t -1 where I is the incident laser intensity and I.sub.t is the threshold intensity for maintaining a plasma at zero velocity.